A Quartz Crystal Microbalance (QCM) system utilizes the piezoelectric effect of a quartz crystal. In such a system a quartz crystal that is placed between two electrodes, which are connected to an AC-potential, begins to oscillate if the frequency of the AC-potential is close to the resonance frequency of the oscillation mode for the quartz crystal. The resonance frequency of the quartz crystal is a function of many parameters, such as temperature, pressure, cut angel of the crystal, mechanical stress and thickness of the crystal. The resonance frequency is inversely proportional to the square root of the thickness of the crystal as described by the Saurbrey equation:
      Δ    ⁢                  ⁢    f    =            -                        2          ⁢                      f            2                                    ρ          ⁢                                          ⁢          v                      ⁢                  Δ        ⁢                                  ⁢        m            A      where f is the resonance frequency, ρ the density of quartz, v the shear wave velocity in quartz, A the electrode area and Δm the sample mass. Typical resonance frequencies used in liquid applications range from 1 MHz to 50 MHz. The crystal is normally AT-cut with a circular or quadratic shape with a diameter of approximately 5-10 mm. The electrodes are normally of gold on both sides, but other metals are not unusual. The electrodes are very thin compared to the quartz crystal plate and can therefore be considered as part of the crystal plate. When material is added to or removed from one of the electrodes, it becomes thicker or thinner, i.e. the associated weight of the electrode changes. As a consequence of the mass change of the electrode, the resonance frequency of the crystal plate will either decrease or increase and hence the change of resonance frequency can be measured to detect the mass change of the electrode. The mass resolution of a QCM system can be as low as 1 pg/cm2, corresponding to less than 1% of a monolayer of hydrogen.
A typical QCM piezoelectric system comprises a sensor unit, a sample insertion unit, a frequency counter, and signal presentation equipment and buffer and waste containers. A sample, which can contain any chemical substance of interest, is introduced into the sensor unit by the sample insertion unit. The sensor unit contains a piezoelectric resonator, a sensor chamber, flow channels to and from the chamber and an oscillating circuit. The sample induces an interaction with the piezoelectric sensor surface, which can in turn be observed by monitoring the oscillating characteristics of the crystal plate, e.g. by measuring changes in the piezoelectric resonator frequency. The resonator utilised by the sensor unit includes a crystal plate, which is a plane piezoelectric crystal. The crystal plate is provided with electric contact areas for an electrode and a counter electrode on its surface, which electrodes are connectable to a signal source (e.g. an alternating voltage source) as well as to a measurement device. For measuring, the piezoelectric crystal plate is on one side brought into contact with the liquid sample to be examined. The crystal responds to the accumulation of the mass of the substance to be detected or to a change in the physical properties of the sample by altering its resonance frequency and/or oscillation amplitude.
Piezoelectric sensors can be used for analysis of the viscosity of a liquid sample and are particularly suitable for studying chemical and biochemical interactions. If a piezoelectric sensor is to be used for the latter purpose, the electrode that is to be exposed to the sample is provided with a specific surface coating, which will interact with the sample. Such surface coated piezoelectric sensors can be used within for example surface science, biotechnology research and pharmaceuticals development. Other applications can be as sensor for detection of hazardous gases or substances such as environmental contaminants, biochemical warfare agents and illicit drugs, e.g. narcotic substances or performance improving drugs (doping). A third area for application of the technology is health diagnostics, where the sensor can be used for examining patients for different diseases by analysis of human blood or other body fluids. The use of piezoelectric resonators in these areas has up to now not gained a commercial acceptance, due to the drawbacks of the hitherto available systems.
The crystal plates utilised by piezoelectric resonators are usually small oscillator quartz crystal plates. However, due to the structure of the small plates, the resonating behaviour of the resonator may be impaired, i.e. by mechanical tension in the small plates caused by the holding carriers or by contacting with electric lines.
An example of a small oscillator quartz crystal plate that is fixed in a holding carrier is described in GB 2 154 058 A. The small oscillator quartz plate is provided with electrodes deposited on its surface and is spatially fixed between two holding clamps and electrically contacted. A disadvantage of this arrangement is that even with the least exerted force, acting on the two parts, which are spaced apart, a mechanical tension can be generated in the small plate glued between parts, thereby influencing its resonance behaviour.
As already mentioned piezoelectric resonators are advantageously used as active sensor elements, e.g. for detecting a substance in a medium, for instance, for measuring the concentration of the substance in a liquid. When working with liquids, the oscillator quartz to be electrically contacted has also to be insulated liquid-tight against the to-be-examined liquid in order to prevent electric short circuits. Such a sensor is described in EP 453 820 B1. This sensor provides a small oscillator quartz plate, which is clamped on both sides between two silicon seals and, in addition, is contacted to conductive adhesive substances. However, the use of conductive adhesives has the consequence that the electric contact cannot be disconnected, which, for instance, makes replacing the small oscillator quartz plate impossible or at least requires great manual skill. Moreover, the silicon seals surrounding the small oscillator plates on all sides have to be made with great precision in order to prevent deformations in the small plate.
These known sensors for measuring concentrations or reactions of a substance in liquids and for determining the physical properties of liquids operating on the basis of piezoelectric resonators have in brief the following drawbacks:
The small oscillator quartz plates are glued to or clamped in a holding means, which can impair the resonance behaviour of the resonator itself due to mechanical tensions. Moreover, glued and clamped electric contacts on the surface of small oscillator quartz plates are not totally reliable, in particular, when employed for measuring concentrations of substances in liquids. Precautions must be taken in order to avoid short circuits. A stable and uniform quality of the contacting is not possible by this means.
In the known cases, the piezoelectric resonators are provided with contact areas, which are to be connected to an electrical oscillation circuit and respectively to a measuring system, for electrical contacting on both its front side as well as on its rear side. The integration of the piezoelectric resonator in a casing with respective electric contacting is difficult and time consuming. Good stable quality of the electric properties, in particular, when using a piezoelectric resonator in a holding means, which permits bringing the small oscillator plate into contact with a sample to on one side, is impossible.
Holding means of this type are also called flow-through cells, which are a unit, in which the already electrically contacted small oscillator quartz plate is connected to the connecting electrodes. Furthermore, defined supply and drain channels are provided via which the sample can be selectively supplied to the piezoelectric resonator and drained again. For flow-through cells of this type, the user needs much time and manual skill in replacing the crystal. In particular, in the field of biosensors, replacement often becomes necessary, because each different substance to be detected requires a specially prepared flow-through cell. In an attempt to overcome these problems the resonator has been integrated in a flow-through cell, as described in U.S. Pat. No. 6,196,059. However, even though such an integrated flow-through cell may be easy to handle, it has to be manufactured with high precision. Another drawback is that the surface coating of the electrode area has to be performed before gluing the resonator to the flow cell and the entire flow cell must be disposed of after use. In addition, there is a risk that the gluing process may interact with the surface coating and disable the desired functionality of the surface coating.
The object of the present invention is to provide a piezoelectric sensor arrangement that gives reliable analysis results, has inexpensive disposable parts and is easy to handle.